The characteristics of small towns and semi-rural areas lend themselves well to investigations of the network characteristics of active, not-in- treatment drug users, and the relationship of drug user network structure to HIV transmission through drug related (injection and sexual) risk behavior. This project will allow us to 1) identify the relationships between drug user network structure and HIV risks, 2) identify the relationships between structure and risk over time, and then 3) identify the relationships between structure and risk over time in relation in HIV transmission. Thr proposed project is both a stand alone R01, and one half of a two site IRPG. The IPRG elements will allow us to 1) compare small town and urban differences in network structure and risk, 2) explore network typology systems and risk, 3) identify the effects of HIV positives and illness on network structure, 4) allow transmission model building, and 5) allow us to evaluate methods for the rapid assessment of network structure on HIV risk. Pooling data form both projects will allow us to also examine two different points of the HIV seroprevalence continuum. We will both cross-cultural and intracultural comparisons of the drug and sexual practices (risk profiles), and the proportions of uniplex and multiplex relationships within networks (including measures of centrality and connected components). We will describe and compare the drug network typologies found in small town and urban cultural groups. (This analysis will be based on typologies previously described among networks of drug users in Flagstaff AZ.) And we will assess the degree to which the presence of known HIV positive persons and HIV related illness changes the structure of drug user networks, and the potential for HIV transmission. The data will allow us to furnish estimates for assortative and disassortative mixing within the populations studied, which can be used to develop a predictive model for potential HIV transmission. We hypothesize that 1) the network structure of drug users will vary along the HIV-prevalence spectrum.. Specifically, HIV-positive persons become more central in networks as prevalence increases; 2) network stability represents a barrier to the spread of HIV-infection 3) Persons from a given network recruited early in the study are likely to be more central in their network; 4) In an area of moderate HIV-prevalence, a small number of respondents will provide accurate information about network structure. In areas of low prevalence, more extensive sampling is required, and 5) The cultural composition of a drug-using network will be an important factor in facilitating (or inhibiting) viral transmission. Specifically, if cultural features of a network inhibit interaction with other groups, the likelihood of transmission is diminished.